The technical field relates to a ported internal combustion engine. More specifically the technical field relates to a ported internal combustion engine that incorporates low-tension compression seals to achieve high BMEP (brake mean effective pressure) operation. The technical field also relates to an opposed-piston, compression ignition engine in which low-tension compression seals are mounted to the opposed pistons so as to minimize port bridge wear during all phases of engine operation while also limiting blow-by during combustion.
A ported internal combustion engine is an internal combustion engine having a cylinder with one or more ports formed therein for the passage of air into and/or out of the bore. For example, the cylinder of a traditional opposed-piston engine includes exhaust and inlet ports cast or machined into the cylinder near respective exhaust and inlet ends of the cylinder liner. Pistons disposed crown-to-crown in the liner's bore traverse the ports while moving through respective bottom dead center (BDC) positions. Rings are mounted to the pistons to maintain a seal between the pistons and the liner bore, which reduces the passage of combustion gasses between the pistons and the bore (blow-by). The rings are heavily tensioned against the bore to accommodate bore/liner distortion caused by thermal and mechanical stresses. Each piston and its rings traverse a respective port twice during every complete engine cycle. The heavy tension forces the outer surfaces of the rings into a high frictional engagement with the bore and with the port bridges, especially where the rings contact the edges of the port openings. As a consequence, repeated transits by the rings over the ports result in excessive and uneven port bridge wear, and, ultimately, early ring failures. The exhaust piston rings suffer particularly heavy damage due to the high temperatures encountered at the exhaust port.
As a result of low durability due to bridge wear, traditional ported engines have had very limited acceptance in the markets for land, air, and marine engines. Measures have been proposed to reduce the complex frictional interface between the piston rings and the port bridges. One such step includes excessive lubrication of the piston/bore interface. However, oil consumption in these cases is typically about 2% of fuel consumption, as compared to portless engines in which oil consumption is typically about 0.1% of fuel consumption. Such high oil consumption is not acceptable under modern emission standards. Other measures include rounding and/or ramping the outer edges of the rings, beveling the edges of the port openings, and customizing the shapes of the port openings. However, these solutions add to manufacturing costs and will continue to have only limited effectiveness so long as the rings are heavily tensioned.
Ported engine constructions have been proposed which incorporate pistons with axially symmetrical construction and coolant structures for cooling pistons and cylinder liners that reduce or eliminate bore/liner distortions throughout engine operation. Because these cooling designs maintain circularity of the bore/piston interface longitudinally of the cylinder throughout engine operation, they eliminate the need for heavily-tensioned rings. An example of such a design in an opposed piston engine construction is found in commonly-owned U.S. Pat. No. 7,360,511, issued Apr. 22, 2008. Accordingly, we have realized that ported internal combustion engines in which circularity of the bore/piston interface is maintained during all phases of engine operation are well-suited for low-tension piston compression seals which substantially reduce wear on port bridges while also limiting blow-by.